Pathologic angiogenesis occurs under many conditions and is thought to be induced by local ischemia. Diseases in which angiogenesis is thought to play a critical role in the underlying pathology include: ocular diseases such as diabetic retinopathy, retinopathy or prematurity and age-related macular degeneration; vascular diseases such as ischemic heart disease and atherosclerosis; chronic inflammatory disorders such as psoriasis and rheumatoid arthritis; and solid tumor growth. A recent review primarily focuses on the role or RTKs in tumor angiogenesis. Shawver, L. K., et al., Receptor Tyrosine Kinases as Targets for Inhibition of Angiogenesis, Drug Discovery Today (Elsevier Science Ltd.), 2(2):50 (1997). The review primarily addresses the role of growth factors and their receptor tyrosine kinases (RTKs) in the regulation of microvessel physiology as they relate to the angiogenic process. New blood vessel growth is required for the growth and metastasis of solid tumors. The significance of angiogenesis in human tumors has been highlighted by recent studies that relate the angiogenic phenotype to patient survival. These studies found that the number of microvessels in a primary tumor has prognostic significance in breast carcinoma, bladder carcinomas, colon carcinomas and tumors of the oral cavity. Anti-angiogenic agents potentially have broad applications in the clinic. Id. See, also, Herz, Jeffrey M., et al., Molecular Approaches to Receptors as Targets for Drug Discovery, J. of Receptor & Signal Transduction Research, 17(5):671 (1997).
RTKs (also known as growth factor receptors) play an important role in many cellular processes. All of these molecules have an extracellular ligand-binding domain. Upon ligand binding, receptors dimerize, the tyrosine kinase is activated and the receptors become autophosphorylated. Ulrich, A., et al., Cell, 61:203 (1990). The cascade triggered by RTK activation modulates cellular events, determining proliferation, differentiation and morphogenesis in a positive or negative fashion. Disturbances in the expression of growth factors, their cognate RTKs, or constituents of downstream signaling pathways are commonly associated with many types of cancer. Gene mutations giving rise to altered protein products have been shown to alter the regulatory mechanisms influencing cellular proliferation, resulting in tumor initiation and progression. Shawver, L. K., et al., Receptor Tyrosine Kinases as Targets for Inhibition of Angiogenesis, DDT (Elsevier Science Ltd.), 2(2):50 (1997).
One strategy for interfering with receptor signaling is to inhibit ligand binding. This can be accomplished with specific receptor-binding antagonists such as ligand fragments, or with nonspecific antagonists such as suramin, with neutralizing antibodies to either the ligand or receptor, or with an excess of soluble receptor or ligand-binding protein, which will sequester the ligand. A second strategy for interfering with receptor signaling is to block signal transduction by overexpression of a dominant-negative receptor. Because receptor kinases typically dimerize to induce signal transduction through transphosphorylation, prevention of receptor dimerization due to overexpression of kinase-deficient receptors will attenuate activation of signaling. Receptors can be made kinase-deficient by introduction of a point mutation in amino acids critical for kinase function, or deletion of the kinase or entire cytoplastic domain. Another strategy for understanding receptor function involves depleting the receptor protein. This can be accomplished by the introduction of exogenous agents such as antisense oligonucleotides, antisense RNA, or ribozymes, all of which lead to degradation of the receptor mRNA and gradual depletion of the protein in the cell. Id.
The profound cellular effects mediated by tyrosine kinases and phosphotyrosine phosphatases have made them attractive targets for the development of new therapeutic molecules. It is known, for example, that the overexpression of tyrosine kinases, such as HER2, can play a decisive role in the development of cancer and that antibodies capable of blocking the activity of this enzyme can abrogate tumor growth. Slamon, D. J., et al., Science, 235:177 (1987); Drebin, et al., Oncogene 2:387 (1988). More recently, attempts have been made to identify small molecules which act as tyrosine kinase inhibitors. For example, bis monocyclic, bicyclic or heterocyclic aryl compounds (PCT WO 92/20642), vinylene-azaindole derivatives (PCT WO 94/14808) and 1-cycloproppyl-4-pyridyl-quinolones (U.S. Pat. No. 5,330,992) have been described generally as tyrosine kinase inhibitors. Styryl compounds (U.S. Pat. No. 5,217,999), styryl-substituted pyridyl compounds (U.S. Pat. No. 5,302,606), certain quinazoline derivatives (EP Application No. 0 566 266 A1), seleoindoles and selenides (PCT WO 94/03427), tricyclic polyhydroxylic compounds (PCT WO 92/21660) and benzylphosphonic acid compounds (PCT WO 91/15495) have been described as compounds for use as tyrosine kinase inhibitors for use in the treatment of cancer.
The availability of a novel human receptor tyrosine kinase species which is clearly implicated by a functional connection to disease will be ideal for such drug screening as well as diagnosis, study, prevention, and treatment of pathophysiological disorders related to the biological molecule.